234 BIOLOGY AND ITS MAKERS 
out, and the paleontological records are searched, until the 
evidence from both sources establishes the fact that the organ 
or organism under study is but the summation of modifica- 
tions and elaborations of a relatively simple primordial. ‘This 
point settled, physiology is called upon to complete the story. 
Have the functions remained the same through the scries? 
or have they undergone a series of modifications, differentia- 
tions, and improvements more or less parallel with the mor- 
phological series ?” 
Since physiology is an experimental science, all questions 
of this nature must be investigated with the help of experi- 
ments. Organisms undergoing development have been sub- 
jected to changed conditions, and their responses to various 
forms of stimuli have been noted. In the rise of experimental 
embryology we have one of the most promising of the recent 
departures from the older aspects of the subject. The results 
already attained in this attractive and suggestive field make 
too long a story to justify its telling in this volume. Roux, 
Herbst, Loeb, Morgan, E. B. Wilson, and many others have 
contributed to the growth of this new division of embryology. 
Good reasons have been adduced for believing that qualitative 
changes take place in the protoplasm as development pro- 
ceeds. And a curb has been put upon that “great fault of 
embryology, the tendency to explain any and every operation 
of development as merely the result of inheritance.” It has 
been demonstrated that surrounding conditions have much 
to do with individual development, and that the course of 
events may depend largely upon stimuli coming from with- 
out, and not exclusively on an inherited tendency. 
Cell-Lineage.— Investigations on the structural side have 
reached a high grade of perfection in studies on cell-lineage. 
The theoretical conclusions in the germ-layer theory arc 
based upon the assumption of identity in origin of the differ- 
ent layers. But the lack of agreement among observers, cspe- 
